text: Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O). Small quantities of oxygen were released by geological and biological processes, but did not build up in the atmosphere due to reactions with reducing minerals. 
  Oxygen began building up in the atmosphere at approximately 1.85 Ga. At current rates of primary production, todays concentration of oxygen could be produced by photosynthetic organisms in 2,000 years. In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O attained were less than 10% of todays and probably fluctuated greatly.
  The increase in oxygen concentrations had wide ranging and significant impacts on life. Most significantly, the rise of oxygen caused a mass extinction of anaerobic microbes and paved the way for multicellular life.